Fingerprint sensors are used to capture the fingerprint images. The fingerprint images may be used for identification purpose. For example, the captured fingerprint images may be searched in a database in order to match with pre-stored fingerprint images.
There are several ways of capturing fingerprint images, including optical, ultrasonic, passive capacitance, active capacitance, etc. In the passive capacitance and active capacitance methods, capacitance sensors use principles associated with capacitance in order to form fingerprint images. In these methods of imaging, sensor arrays are used. Each of the sensing electrodes in a sensor array acts as one plate of a parallel-plate capacitor. The dermal layer (which is electrically conductive) of the finger to be sensed acts as the other plate, and the non-conductive epidermal layer of the finger acts as a dielectric layer. The differences between the capacitance values of the sensing electrodes in the array reveal the ridges and valleys in the fingerprint.
A conventional fingerprint sensor includes a sensor die having a plurality of surface bond pads at a top surface. The bond pads are bonded to printed circuit boards etc. through wire bonding. The fingerprint sensor may further include a sensor array at the top surface, wherein the sensor array includes a plurality of sensing electrodes. The sensor die is molded in a molding compound, which covers and protects the bond wires and the sensor array.